The present invention relates generally to the field of laser diodes, and more particularly to architecture for short-wavelength nitride based laser diode arrays.
Short-wavelength nitride based laser diodes provide smaller spot size and a better depth of focus than red and infrared (IR) laser diodes for laser printing operations and other applications. Single-spot nitride laser diodes have applications in areas such as optical storage.
Laser diode arrays are desirable for application to high speed laser printing. Printing at high speeds and at high resolution requires laser arrays due to the fundamental limits of polygon rotation speed, laser turn-on times and laser power. Laser diode arrays have previously been employed using red and infrared laser diode structures. Dual-spot red lasers and quad-spot infrared lasers have been used for laser printers.
Laser diodes based on higher bandgap semiconductor alloys such as AlGaInN have been developed. Excellent semiconductor laser characteristics have been established in the near-UV to violet spectrum, principally by Nichia Chemical Company of Japan. See for example, A. Kuramata et al., xe2x80x9cRoom-temperature CW operation of InGaN Laser Diodes with a Vertical Conducting Structure on SiC Substratexe2x80x9d, Japanese Journal of Applied Physics, Vol. 37, L1373 (1998), S. Nakamura et al., xe2x80x9cCW Operation of InGaN/GaN/AlGaN-based laser diodes grown on GaN substratesxe2x80x9d, Applied Physics Letters, Vol. 72(6), 2014 (1998) and S. Nakamura and G. Fasol, xe2x80x9cThe Blue Laser Diode-GaN based Light Emitters and Lasersxe2x80x9d, (Springer-Verlag, 1997) all of which are incorporated by reference in their entirety.
Extension of dual-spot lasers to shorter wavelengths enables printing at higher resolution. However, the architecture for short-wavelength laser diode arrays needs to be different when nitride based laser diodes are used in arrays because mirrors need to be formed by dry etching instead of cleaving and nitride based devices are mostly grown on insulating substrates such as sapphire.
Architectures using insulating substrates allow the economical construction of nitride based quad-spot diode laser and surface-emitting dual-quad-spot laser diode arrays. Currently, most advanced nitride based single laser structures are grown on insulating sapphire (Al2O3) substrates. The use of insulating substrates for laser diode arrays presents a special problem in providing electrical contacts for the laser diodes. In contrast to the situation where conducting substrates are used, insulating substrates cannot provide a common contact for all laser diodes in an array. Hence, providing electrical contacts to laser diode arrays on insulating substrates requires the use of special architectures.
Dual spot and quad spot laser diodes built on an insulating substrate can be electrically contacted using an architecture with surface contacts for both anode and cathode. Two laser diodes may share a common n-contact or p-contact. Alternatively, each laser diode may have separate n-and p-contacts. Providing separate contacts for each laser diode greatly reduces electrical and thermal crosstalk but complicates the laser diode architecture. In quad spot laser diodes, two laser diodes may be aligned at an angle with respect to the other two laser diodes to achieve further reduction in electrical and particularly thermal crosstalk if necessary.
Alternatively, laser diodes built on an insulating substrate as an array may be contacted using multiple buried layers isolated from each other by blocking layers of opposite conductivity or by insulating layers. This allows good isolation of the conducting layers while still maintaining good conductivity. Alternating layers of opposite conductivities form p-n junctions that are reverse-biased under forward bias operation of the laser diode array. As a result, a buried isolated current channel is produced for each laser diode in the laser array. Alternating doped layers with insulating layers also forms a buried isolated current channel for each laser diode in the laser array. The insulating layers provide electrical blocking between the doped layers to isolate the current channel. Blocking layers may be epitaxially grown.
Since it is very difficult to obtain high quality mirror facets by cleaving because of the cleave plane mismatch between GaN and Al2O3, laser mirrors for laser diodes on insulating substrates are most often obtained by using either dry-etched vertical facets (i.e. chemically assisted ion -beam etching) or by integrating a distributed Bragg reflecting mirror into the laser device structure.
The ability to use insulating substrates for short wavelength nitride based lasers by employing special architectures offers a significant economic savings for laser diode array structures as well as allowing use of proven techniques for their manufacture.
Thus, the present invention and its various embodiments provide numerous advantages as will be described in further detail below.